1
This paper appeared in In Tager-Flusberg, H, (ed) Neurodevelopmental Disorders.
MIT Press (1999)







The extreme-male-brain theory of autism



Simon Baron-Cohen


Departments of Experimental Psychology and Psychiatry
University of Cambridge, Downing St, Cambridge CB2 3EB, UK

2


Acknowledgements: I am grateful for support from the MRC (UK), the Wellcome Trust, and the Gatsby
Foundation, during the preparation of this work. Parts of this chapter are reprinted from Baron-Cohen and
Hammer (in press a), and Baron-Cohen and Swettenham (in press), with permission.

3
1. Autism

Autism is widely regarded to be the most severe of the childhood psychiatric conditions
(Rutter, 1983; Frith, 1989; Baron-Cohen, 1995). It is diagnosed on the basis of abnormal
social development, abnormal communicative development, and the presence of narrow,
restricted interests, and repetitive activity, along with limited imaginative ability (DSM-
IV, 1994). Such children fail to become social, instead remaining on the periphery of any
social group, and becoming absorbed in repetitive interests and activities, such as
collecting unusual objects or facts. It is a tragedy for their families who work tirelessly to
attempt to engage with and socialize their child, mostly with very limited results.

In this chapter, I begin by summarizing psychological findings from studies of autism. A
brief review of genetic evidence appears next, as a bridge into the next section, where a
recent notion is introduced: the “male brain”. Evidence for biologically-based
psychological sex differences is presented, and the “male brain” is defined. Finally, I
relate this notion to autism, summarizing our new theory (Baron-Cohen and Hammer,
1996a) that autism is an extreme form of the male brain. This theory makes a number of
predictions possible, and the current evidence relevant to these predictions is presented.

2. Psychological theories of autism

In this section, evidence for three psychological theories is reviewed: the mindblindness
theory, the central coherence theory, and the executive dysfunction theory.

4
The mindblindness theory

Our early theory of autism suggested that the social and communicative abnormalities in
this syndrome could be the result of an impairment in the development of a “theory of
mind”, or the capacity for “mindreading”. This is defined as the ability to attribute mental
states to oneself and others, and make sense of and predict behaviour on the basis of
mental states. This is held to be important to autism simply because it is arguably the
main way in which the normal individual succeeds in understanding and participating in
social relationships and communication.

Wimmer and Perner (1983) devised an elegant paradigm to test when normally
developing children show evidence of possessing a theory of mind - specifically, when
they are aware of another person’s beliefs. The child was presented with a short story,
with the simplest of plots. The story involves one character not being present when an
object is moved, and therefore not knowing that the object is in a new location. The child
being tested is asked where the character thinks the object is. Wimmer and Perner called
this the False Belief test, since the focuses on the subject's ability to infer a story
character's mistaken belief about a situation. These authors found that normal 4 year olds
correctly infer that the character thinks the object is where the character last left it, rather
than where it actually is. This is impressive evidence for the normal child's ability to
distinguish between their own knowledge (about reality) and someone else's false belief
(about reality).

5
When this test was given to a sample of children with autism, with mild degrees of
mental retardation, a large majority of them 'failed' this test by indicating that the
character thinks the object is where it actually is (Baron-Cohen, Leslie, and Frith, 1985).
That is, they appeared to disregard the critical fact that, by virtue of being absent during
the critical scene, the character's mental state would necessarily be different to the child's
own mental state. In contrast, a control group of children with Down Syndrome, with
moderate degrees of mental retardation, passed this test as easily as the normal children.
The implication was that the ability to infer mental states may be an aspect of social
intelligence that is relatively independent of general intelligence (Cosmides, 1989), and
that children with autism might be specifically impaired in the development of a theory
of mind.

Of course, simply failing one test would not necessarily mean that children with autism
lacked awareness of the mind. There might be many reasons for failure on such a test.
(Interestingly, control questions in the original procedure ruled out memory, or language
difficulties, or inattention as possible causes of failure). The conclusion that children with
autism are indeed impaired in this domain only becomes possible because of the
convergence of results from widely differing experimental paradigms. These are
reviewed in detail in an edited volume (Baron-Cohen, Tager-Flusberg, and Cohen, 1993)
and for that reason are only briefly summarized here, next.

Summary of results from studies of mind-reading in children with autism 1

1 In the following list of studies, all of the tests mentioned are at the level of a normal 4 year old child.

6

The majority of children with autism
(i) are at chance on tests of the mental-physical distinction (Baron-Cohen, 1989a). That
is, they do not show a clear understanding of how physical objects differ from thoughts
about objects. For example, when asked which can be touched: a biscuit, or a thought
(about a biscuit), young normal 3 year olds rapidly identify the former, whereas most
children with autism respond at chance levels.

(ii) They also have an appropriate understanding of the functions of the brain, but have a
poor understanding of the functions of the mind (Baron-Cohen, 1989a). That is, they
recognize that the brain's physical function is to make you move and do things, but they
do not spontaneously mention the mind's mental function (in thinking, dreaming,
wishing, deceiving, etc.,). Again, contrast this with normal 3 year old children who do
spontaneously use such mental state terms in their descriptions of what the mind is for
(Wellman and Estes, 1983).

(iii) Most children with autism also fail to make the appearance-reality distinction
(Baron-Cohen, 1989a), meaning that, in their description of misleading objects (like a red
candle in the shape of an apple), they do not distinguish between what the object looks
like, and what they know it really is. For example, the normal 4 year old child will say of
an ambiguous object, when asked what it looks like, and what it really is, that “It looks
like an apple, but really it’s a candle made of wax” (Flavell, Flavell, and Green, 1983). In

7
contrast, children with autism tend to refer to just one aspect of the object (e.g., saying
“It looks like an apple, and it really is an apple”).

(iv) Most children with autism fail a range of first-order false belief tasks, of the kind
described in the previous section (Baron-Cohen et al, 1985, 1986; Perner, Frith, Leslie,
and Leekam, 1989; Swettenham, 1996; Reed and Petersen, 1990; Leekam and Perner,
1991). That is, they show deficits in thinking about someone else’s different beliefs.

(v) They also fail tests assessing if they understand the principle that "seeing leads to
knowing" (Baron-Cohen and Goodhart, 1994; Leslie and Frith, 1988). For example,
when presented with two dolls, one of whom touches a box, and the other of whom looks
inside the box, and when asked "Which one knows what's inside the box?", they are at
chance in their response. In contrast, normal children of 3-4 years of age correctly judge
that it is the one who looked, who knows what's in the box.

(vi) Whereas normally developing children are rather good at picking out mental state
words (like "think", "know", and "imagine") in a wordlist that contains both mental state
and non-mental state words, most children with autism are at chance (Baron-Cohen,
Ring, Moriarty, Shmitz, Costa, and Ell, 1994). In contrast, they have no difficulty in
picking out words describing physical states.

(vii) Nor do most children with autism produce the same range of mental state words in
their spontaneous speech (Tager-Flusberg, 1992; Baron-Cohen et al, 1986). Thus, from

8
about 18-36 months of age, normally developing children spontaneously use words like
“think”, “know”, “pretend”, “imagine”, “wish”, “hope”, etc., and use such terms
appropriately (Wellman, 1990). In contrast, such words occur less frequently, and are
often even absent, in the spontaneous speech of children with autism.

(viii) They are also impaired in the production of spontaneous pretend play (Baron-
Cohen, 1987; Wing, Gould, Yeates, and Brierley, 1977; Lewis and Boucher, 1988).
Pretend play is relevant here simply because it involves understanding the mental state
of pretending. The normal child of even 2 years old effortlessly distinguishes between
when someone else is acting veridically, versus when they are “just pretending” (Leslie,
1987). Sometimes mommy is actually eating (putting a real spoon with real food into her
mouth), whilst at other times mommy is just pretending to eat (holding a pen to her lips,
and making funny slurping noises, in between her smiles).

Young normal children make rapid sense of such behaviour, presumably because they
can represent the latter case as being driven by the mental state of “pretending”. They
also spontaneously generate examples of pretence themselves, and do not show any
confusion as they switch back and forth between pretence (the mental world), and reality
(the physical world). In contrast, most children with autism produce little pretence, and
often appear confused about what pretence is for, and when someone is or is not
pretending.

9
(ix) Whilst they can understand simple causes of emotion (such as reactions to physical
situations), the majority of children with autism have difficulty understanding more
mentalistic causes of emotion (such as beliefs) [Baron-Cohen, 1991a; Baron-Cohen,
Spitz, and Cross, 1993]. For example, they can understand that if Jane actually falls over
and cuts her knee, she will feel sad, and that if John actually gets a present, he will feel
happy. But they are poor at understanding that if John thinks he's getting a present (even
if in reality he is not), he will still feel happy. In contrast, normal 4 year old children
comprehend such belief-based emotions.

(x) Most children with autism also fail to recognize the eye-region of the face as
indicating when a person is thinking and what a person might want (Baron-Cohen and
Cross, 1992; Baron-Cohen, Campbell, Karmiloff-Smith, Grant, and Walker, 1995).
Children and adults without autism use gaze to infer both of these mental states.

For example, when presented with pairs of photos like those in Figure 1, normal 3-4 year
olds easily identify the person looking upwards and away as the one who is thinking.
Children with autism are less sure of this. And when shown a display like the one in
Figure 2, normal 4 year olds identify the candy that Charlie is looking at as the one he
wants. Children with autism mostly fail to pick up that gaze can be an indicator of what a
person might want.

insert Figures 1 and 2 here

10
In addition:

(xi) Many children with autism fail to make the accidental-intentional distinction
(Phillips, 1993). That is, they are poor at distinguishing if someone "meant" to do
something, or if something simply happened accidentally.

(xii) They also seem unable to deceive (Baron-Cohen, 1992; Sodian and Frith, 1992), a
result that would be expected if one was unaware that people's beliefs can differ and
therefore can be manipulated. In contrast, normal children of 4 begin to be quite adept at
lying, thus revealing their awareness of the mental lives of others.

(xiii) Most children with autism also have disproportionate difficulty on tests of
understanding metaphor, sarcasm, and irony - these all being statements which cannot be
decoded literally, but which are only meaningful by reference to the speaker’s intention
(Happe, 1994). An example would be understanding the phrase “the drinks are on the
house”, which one adult with autism (of above average IQ) could only interpret literally.
This suggests that children with autism are aware of the physical (the actual words
uttered), but are relatively unaware of the mental states (the intentions) behind them.

(xiv) Indeed, most children with autism fail to produce most aspects of pragmatics in
their speech (reviewed in Baron-Cohen, 1988; and Tager-Flusberg, 1993), and fail to
recognize violations of pragmatic rules, such as the Gricean Maxims of conversational
cooperation (Surian, Baron-Cohen, and Van der Lely, 1996). For example, one Gricean

11
Maxim of conversation is “Be relevant”. If someone replies to a question with an
irrelevant answer, normal young children are very sensitive to this pragmatic failure, but
most children with autism are not. Since many pragmatic rules involve tailoring one's
speech to what the listener expects, or needs to know, or might be interested in, this can
be seen as intrinsically linked to a sensitivity of another person’s mental states.

(xv) Crucially, most children with autism are unimpaired at understanding how physical
representations (such as drawings, photos, maps, and models) work, even while they
cannot understand mental representations (such as beliefs) [Charman and Baron-Cohen,
1992, 1995; Leekam and Perner, 1991; Leslie and Thaiss, 1992]. To the extent that both
types of task require understanding of representation, this suggests there is something
special about understanding mental representations that causes problems in autism.

(xvi) They are also unimpaired on logical reasoning (about the physical world) even
though they have difficulty in psychological reasoning (about the mental world) [Scott
and Baron-Cohen, 1996).

This long list of experiments provides strong evidence for children with autism lacking
the normal understanding of mental states. For this reason, autism can be conceptualized
as involving degrees of mindblindness (Baron-Cohen, 1990, 1995).

It is important to mention that a small minority of children or adults with autism pass
first-order false belief tests. (First-order tests involve inferring what one person thinks).

13
Scale (Frith, Happe, and Siddons, 1994). In the next sections, we turn to consider the
neural and developmental origins of this cognitive deficit.

The brain basis of theory of mind

One possibility arising from these studies is that there may be a particular part of the
brain which in the normal case is responsible for our mindreading ability, and which is
specifically impaired in autism. If this view is correct, the assumption is that this may be
for genetic reasons, since autism appears to be strongly heritable (see Santangelo and
Folstein, chapter 17, this volume). The idea that the development of our theory of mind is
under genetic control in the normal case is consistent with evidence from cross-cultural
studies: Normally developing children from markedly different cultures seem to pass
tests of ‘mind-reading’ at roughly the same ages (Avis and Harris, 1991).

Quite which parts of the brain might be involved in this is not yet clear, though candidate
regions include right orbito-frontal cortex, which is active when subjects are thinking
about mental state terms during functional imaging using SPECT (Baron-Cohen, Ring, et
al, 1994); and left medial frontal cortex, which is active when subjects are drawing
inferences about thoughts whilst being PET scanned (Fletcher, Happe, Frith, Baker,
Dolan, Frackowiack, and Frith, 1995; Goel, Grafman, Sadato, and Hallett, 1995). Other
candidate regions include the superior temporal sulcus and the amygdala (for reasons
explained later). These regions may form parts of a neural circuit supporting theory of
mind processing (Baron-Cohen and Ring, 1994).

15

This was an important discovery because joint attention behaviours are normally well-
developed by 14 months of age (Scaife and Bruner, 1975; Butterworth, 1991), so their
absence in autism signifies a very early-occurring deficit. This was also important
because the traditional mind-reading skills referred to above are mostly those one would
expect to see in a 3-4 year old normal child. Deficits in these areas cannot therefore be
the developmentally earliest signs of autism, since we know that autism is present from at
least the second year of life (Rutter, 1978), if not earlier.

Implicit in the idea of joint attention deficits in autism was the notion that these might
relate to a failure to appreciate other people's point of view (Sigman et al, 1986).
Bretherton, McNew, and Beeghly-Smith (1981) had also suggested joint attention should
be understood as an “implicit theory of mind” - or an implicit awareness of the mental.
Baron-Cohen (1989c,d, 1991c) explicitly argued that the joint attention and mind-reading
deficits in autism were no coincidence, and proposed that joint attention was a precursor
to the development of mind-reading. In that study (Baron-Cohen, 1989c), young children
with autism (under 5 years old) were shown to produce one form of the pointing gesture
(imperative pointing, or pointing to request) whilst failing to produce another form of
pointing (declarative pointing, or pointing to share interest).

This dissociation was interpreted in terms of the declarative form of pointing alone being
an indicator of the child monitoring another person's mental state - in this case, the
mental state of "interest", or "attention". More recent laboratory studies have confirmed

18
Counterfactual syllogistic reasoning tests [eg. All cats bark, Rex is a cat, therefore Rex ?]
(Scott, Baron-Cohen, and Leslie, 1996).

Finally, Happe (in press) reports that some very high functioning people with autism who
pass second-order theory of mind tasks nevertheless fail tasks of central coherence, such
as the homophone task, mentioned earlier. This dissociation implies theory of mind and
central coherence may be relatively independent processes (Frith and Happe, 1994).
Whether both deficits in autism in fact reduce to a more basic deficit is still the subject of
controversy. In sum, a weak form of central coherence theory seems likely to be correct,
disabling individuals with autism from making full use of context. Whether this can
account for islets of ability in autism (and even in Savant Syndrome) remains to be
investigated in detail.

Executive function and autism

This is the third and final area of psychological studies for which claims have been made
of impairments in autism. Executive function is the postulated mechanism which enables
the normal person to shift attention flexibly, inhibit prepotent responses, generate goal-
directed behaviour, and solve problems in a planful, strategic way (see Shallice, 1988;
Baddeley, 1991). The basic idea, developed by Norman and Shallice (1980), is that
without a "central executive", or a "Supervisory Attentional System" as it is also called,
actions are controlled by the environment, such that the organism simply responds to
cues which elicit behaviour. Without a Supervisory Attentional System, action schemas

19
or motor programs 'contend' between themselves for execution. This takes place in a
system known as the Contention Scheduling System. Shallice's notion is that the
Contention Scheduling System is broadly a basal-ganglia function, whilst the
Supervisory Attentional System is basically a frontal lobe function. The Supervisory
Attentional System allows inhibition of routine actions. The claim that the Supervisory
Attentional System is a frontal function derives from the evidence that patients with
frontal lobe damage fail tests of this (or executive) function.

Tests of executive function include the following:
(i) The Wisconsin Card Sorting Test (Milner, 1964) in which the subject has to shift card-
sorting strategies flexibly.
(ii) The Tower of Hanoi (and its modified version, the Tower of London [Shallice,
1982]), in which the subject has to solve problems by planning before acting.
(iii) The Verbal Fluency Test (or F-A-S test: see Perret, 1974) in which the subject has to
generate novel examples of words beginning with a given letter, in a fixed time period.
(iv) The Detour Reaching Test (Diamond, 1991), in which the subject has to inhibit
reaching straight for a visible goal, and instead take a detour route to the goal.

Patients with frontal lobe damage fail on these tasks (reviewed in Shallice, 1988), and so
do people with autism (Rumsey and Hamberger, 1988; Prior and Hoffman, 1990;
Ozonoff, Pennington, and Rogers, 1992; Hughes and Russell, 1993; Hughes, Russell, and
Robbins, 1994). This has led to the conclusion that children with autism might have
frontal lobe damage. One suggestion arising from this is that they might fail theory of

21
impairment, and yet do not lead to autism, it follows that, by itself, an impairment in
executive function cannot explain autism. Note that examples of patients or disorders
which show a double dissociation between executive function and theory of mind would
be the strongest test of the independence of these processes2.

It may be that, as presently construed, the concept of executive function is too broad a
level of analysis. The model suggests this has several component processes (generativity,
attention-shifting, disengaging, etc.,), and it may be that specificity of deficit will be
more apparent at this more fine grain level of analysis. One example of a component
process hypothesis is that in autism there is a deficit in "disengaging from the salience of
reality". However, this cannot be correct in its strong form. This is because there are a
number of studies in which subjects have to do just this, and yet children with autism
pass these tests. These include the following:

(i) visual perspective taking (Baron-Cohen, 1989c, 1991b; Hobson, 1984; Tan and
Harris, 1991). In these tasks, the child has to infer what someone else can see from their
spatial position, even if this is different to what the child currently sees.
(ii) False photograph tests (Leslie and Thaiss, 1992; Leekam and Perner, 1991;
Swettenham, Baron-Cohen, Gomez, and Walsh, 1996). In these tasks, the child has to
infer where something will be in an out-dated photograph of reality, when they know that
reality has been changed such that the object is actually in a new position.

2 A further confound is that many tests of theory of mind involve some attention shifting, and many tests of
executive function involve taking into account one's own mental states, such as one's plans and thoughts.

22
(iii) False map tests (Leslie and Thaiss, 1992; Leekam and Perner, 1991). These tasks test
the same ability as the false photograph task, but using a map instead of a photograph.
(iv) False drawing tests (Charman and Baron-Cohen, 1992). These tasks test the same
ability as the false photograph task, but using a drawing instead of a photograph.
(v) False model tests (Charman and Baron-Cohen, 1995). These tasks test the same
ability as the false photograph task, but using a model instead of a photograph.
(vi) Intellectual realism tests in drawing (Charman and Baron-Cohen, 1993). In these
tasks, the subject is asked to draw an object that is partially occluded - for example,
drawing a coffee mug in which the handle is out of view. Children with autism show
"intellectual realism" at the same mental age as do children without autism (ie: below a
mental age of about 6 yrs old), in that they include the occluded object even though this
is out of view. For example, they draw the handle of the coffee mug, even when this is
not visible. (It is not until after an MA of about 6 years has been achieved that subjects
(with or without autism) show "visual realism", drawing only what they see, not what
they know about). This task is relevant in that if children with autism were `prisoners' of
reality, they should show precocious visual realism, which they do not.
For these reasons, the executive function hypothesis remains in need of considerable
clarification. In addition, it is unlikely that theory of mind is reducible to executive
function. Executive function deficits in autism may instead cooccur with theory of mind
deficits because of their shared frontal origin in the brain. Despite these provisos, the
executive hypothesis of autism is important, since an attraction of the account is its
potential to explain the perseverative, repetitive behaviours in this condition, which are
not accounted for by the theory of mind hypothesis. Perseveration and repetitive

23
behaviours are symptomatic of frontal lobe syndrome in which executive dysfunction is
also seen (Shallice, 1988). On this view, the two cognitive deficits may be separately
responsible for different types of abnormal behaviour.

3. Genetics and autism

Santangelo and Folstein (this volume, chapter 17) provides a thorough review of the
genetics of autism, to which the interested reader is referred. However, as a bridge
between the psychological evidence reviewed above, and the new model of autism
discussed later in the chapter, the key evidence for genetic factors in autism is briefly
summarized here.

Autism (and Asperger Syndrome) appear to be strongly heritable. Here is the heritability
evidence. First, family studies have shown that first degree relatives of people with
autism have a raised risk of autism, compared to population baseline levels (Folstein and
Rutter, 1988). For example, whilst estimates of autism in the general population range
from 1 in 2500, to 1 in 1000 (Wing and Gould, 1979), the sibling risk rate in families
with a child with autism is 3%. This is therefore significantly higher than the population
baseline rate. Such family data could imply an environmental or hereditary cause.
However, twin studies implicate a genetic aetiology more persuasively. The concordance
rate for autism among monozygotic (MZ) twins is as high as 60% whilst the concordance
rate among dizygotic (DZ) twins is no higher than

25
for this notion comes from the long history of research into sex differences in cognition.
This is briefly summarized next.




Evidence that males and females differ in cognition

Some of the key findings (for reviews, see Buffery and Gray, 1972; Kimura, 1992;
Halpern, 1992; McGee, 1979; Geary, 1996) are that (as a group) women are superior to
men on:

(a) language tasks (such as the Verbal Fluency Task - eg: list as many words as you can,
beginning with the letter ‘L’). Females also show a faster rate of language development,
and a lower risk for specific language impairment. (See Hyde and Linn, 1988, on sex
differences in language; and Bishop, 1990, on language disorder);
(b) tests of social judgement (Hall, 1977; Halpern, 1992; Argyle and Cooke, 1976);
(c) measures of empathy and cooperation (Hutt, 1972);
(d) rapid identification of matching items (also known as “perceptual speed”: Kimura,
1992);
(e) ideational fluency (eg: list as many things as you can that are the same colour:
Kimura, 1992);
(f) fine-motor coordination (eg: placing pegs in pegboard holes: Kimura, 1992);

26
(g) mathematical calculation tests (Kimura, 1992);
(h) pretend play in childhood (Hutt, 1972).

In contrast, men (as a group) are superior to women on:
(i) mathematical reasoning, especially geometry and mathematical word problems
(Lummis and Stevenson, 1990; Stevenson et al, 1990; Marshall and Smith, 1987;
Steinkamp et al, 1985; Johnson, 1984; Mills, Ablard, and Stumpf, 1993). Benbow and
Stanley (1980, 1983) for example reports that at high-level mathematics, the male-female
ratio is 13:1;
(ii) the Embedded Figures Task (ie: finding a part within a whole) (Witkin et al, 1971).
(iii) the Mental Rotation Task (ie: imagining how an object will look when it is rotated,
or how a sheet of paper will look when it is folded: Masters and Sanders, 1993;
Kalichman, 1989);
(iv) some (but not all4) spatial skills - mostly Euclidean geometric navigation (Linn and
Petersen, 1985; Gilger and Ho, 1989; Law, Pellegrino, and Hunt, 1993; Voyer, Voyer,
and Bryden, 1995; Witelson, 1976). Spatial superiority in males is even found in
childhood (Kerns and Berenbaum, 1991).
(v) target-directed motor skills, such as guiding or intercepting projectiles - irrespective
of the amount of practice (Kimura, 1992; Buffery and Gray, 1972).

Evidence that the male and female brain are determined prenatally

4 Kimura (1992) for example reports that men are not superior over women on measures of recall of
landmarks from a route.

28
Defining a male and female brain7

Evidence reviewed in the previous section points to the notion that during foetal life,
endocrine factors shape the brain as either:
(a) more developed in terms of “folk psychology” and less developed in terms of “folk
physics”. (Moir and Jessel, 1989, in their popular book, for shorthand call this "the
female brain type"); or
(b) or vice versa ("the male brain type").
Folk psychology is broadly “mindreading”, and folk physics is broadly understanding
physical objects (and this includes mechanical, constructional, mathematical and spatial
skills) (Pinker, in press). In our model, we operationally define the male brain type as an
individual whose folk physics skills are in advance of his or her social folk pscyhology
skills. That is, they show a folk physics>folk psychology discrepancy. This is regardless
of one’s chromosomal sex. Similarly, we will define the female brain type as an
individual whose folk psychology skills are in advance of his or her spatial folk physics
skills. That is, they show a folk psychology>folk physics discrepancy. Again, this is
regardless of one’s sex. Clearly, this suggests that yet other people might have neither the
male nor the female brain type, because their folk psychology skills are roughly equal to
their folk physics skills. We will call this third possibility the "cognitively balanced brain
type". Autism (and Asperger Syndrome), we will argue, are extreme forms of the male

7 This model should not be used to reinforce traditional occupational and economic inequalities between
the sexes. A detailed reading of the model should lead the reader to draw conclusions based on
individuals’ brain type rather than their sex.

29
brain type. That is, the folk physics>folk psychology discrepancy is even larger than in
the normal male brain type. These types of brain are summarized in Figure 3.

insert Figure 3 here

Neural substrates of the male and female brain

Precisely which structures distinguish these two brain types is still controversial (see
Fitch and Dennenberg, 1996, for a review). Kimura (1992) reviews evidence for
differences in cerebral lateralization. In particular, she reviews evidence that at birth, in
the human male foetus, the right hemisphere cortex is thicker than the left: Some reports
also show the corpus callosum is larger in females (De la Coste-Utamsing and Holloway,
1982), though reports are conflicting (Wittelson, 1989, 1991; Habib et al, 1991;
Dennenberg, Kertesz, and Cowell, 1991). Hines (1990) reviews 13 studies and concludes
that in females the corpus callosum is larger, and that this might cause the female
superiority in verbal fluency (as a function of better interhemispheric transfer of
information).


Finally, there is evidence that aspects of folk physics such as spatial ability are affected
by hormonal changes. For example, exposure to androgens prenatally increases spatial
performance in human females and females of other species (Resnick et al, 1986; see
Hines and Green, 1991; and Halpern, 1992), and castration of the rat decreases spatial

31
ear or visual field, relative to right-handers. Thus, he found 82% of right-handers, but
only 62% of left handers, show a right-ear advantage in dichotic listening (verbal) tasks.
Males have a much higher rate of left-handedness than females (Halpern, 1992). Thus,
when Bryden analysed the same data by sex, he found that 81% of males, but only 74%
of females, showed a right ear advantage. He concludes that in general, females have a
more bilateral organization of cognitive abilities than males. Hines (1990) expresses the
same idea differently: the degree of left-hemisphere dominance is greater in males than
females.

Regarding the link between lateralization and folk physics, Benbow (1986) reported an
elevated incidence of left-handedness in children gifted mathematically. Hassler and
Gupta (1993) also found left-handers score higher on a measure of musical talent, and
(replicating the earlier work) show reduced right-ear advantage. In addition, Cranberg
and Albert (1988) reported an elevated incidence of non-right handedness in high level
male chess players. Rosenblatt and Winner (1988) found a very high rate of left-
handedness and ambidextrality in children with exceptional drawing ability. Kimura and
D’Amico (1989) found that non-right handed science students in university have higher
spatial ability than right-handed controls. Sanders, Wilson, and Vandenberg (1982)
found, in their family study, that left-handed men score higher than right-handed men on
spatial tasks (though left-handed women were worse than right-handed women). Indeed,
elevated rates of left-handedness occurs in the those working in the visuo-spatial arts
(Mebert and Michel, 1980; Peterson, 1979), in architecture, and in engineering (Petersen

33
(i) Normal males are superior in spatial tasks compared to normal females, and people
with autism or Asperger Syndrome are even better on spatial tasks, such as the Embedded
Figures Test (Jolliffe and Baron-Cohen, in press).
(ii) There is a strong male bias in the sex ratio of autism or AS.
(iiii) Normal males are slower to develop language than normal females, and children
with autism are even more delayed in language development (Rutter, 1978).
(iv) Normal males are slower to develop socially than normal females, and people with
autism are even more delayed in social development (O’Riordan, Baron-Cohen, Jones,
Stone, and Plaisted, 1996).
(v) Normal females are superior to males on mindreading tasks, and people with autism
or AS are severely impaired in mindreading (see Baron-Cohen et al, 1996).
(vi) Parents of children with autism or AS (who can be assumed to share the genotype of
their child) also show superior spatial abilities and relative deficits in mindreading (i.e., a
marked male brain pattern (Baron-Cohen and Hammer, in press b).
(vii) Normal males have a smaller corpus callosum than normal females, and people with
autism or AS have an even smaller one (Egaas, Courchesne, and Saitou, 1994).
(viii) Left handedness is more common among males, and people with autism or AS
show an elevated incidence of left-handedness: Fein, Humes, Kaplan, Lucci, and
Waterhouse (1984) found an 18% incidence of left-handedness in autism. Satz and
colleagues (Satz, Soper, Orsini, Henry, and Zvi, 1985; Soper, Orsini, Henry, Zvi, and
Schulman, 1986) found a very similar picture: in their autistic sample, 22% were left
handed10.

10 It should be noted though that anomalous handedness is also present in children with general

34
(ix) In the normal population, the male brain is heavier than the female brain, and people
with autism have even heavier brains than normal males (Bailey et al, 1994).
(x) In the normal population, more males are found in mathematical/mechanical/spatial
occupations than females. Parents of children with autism or AS are disproportionately
represented in such occupations (Baron-Cohen, Wheelwright, Bolton, Stott & Goodyer
1996). These occupations all require good folk physics whilst not necessarily requiring
equally developed folk psychological skills.

Evidence from autism inconsistent with the theory

Since males are more strongly lateralized than females, people with autism should show
strong lateralization. Studies looking at lateralization in autism using dichotic listening
tasks and evoked auditory potentials reveal abnormalities, but in the opposite direction to
those predicted by the theory. Thus, Prior and Bradshaw (1979) found children with
autism show no clear right-ear advantage in dichotic listening tasks; and Dawson, Finley,
Phillips, and Galpert (1986) found they did not show the asymmetry of evoked response
to auditory speech, unlike normal controls. The most recent relevant study is a SPECT
neuroimaging investigation of autism reporting a lack of normal hemispheric asymmetry
(Chiron, Leboyer, Leon, Jambaque, Nuttin, and Syrota, 1995). Satz concludes that
children with autism are less strongly lateralized, compared to normal children. This is
not consistent with the extreme male brain theory of autism. However, this may have

developmental delay (irrespective of whether they have autism - see Bishop, 1990). It remains to be seen,
then whether the anomalous handedness in autism is specific to this condition, or secondary to general
developmental delay that is present in two-thirds of children with autism.

35
arisen because these studies looked at lateralization of language, in children with autism
plus significant language delay. It would be interesting for future studies to look for
lateralization of spatial abilities, in cases of ‘pure’ autism or Asperger Syndrome, in order
to test the extreme male brain theory further.



6. Conclusions: the continuum of male and female brain types

An important assumption of the model is that all individuals fall on a continuum as
regards male and female brain types. As stated earlier, we have referred to some
individuals as "cognitively balanced", being equally good at folk physics and folk
psychology. They show no discrepancy. Other individuals are better at folk physics than
they are at folk psychology: this corresponds to the male brain type. People with the male
brain type might show this discrepancy just marginally (the normal male brain type), or
just more than this (a touch of Asperger Syndrome), or more markedly still (frank
Asperger Syndrome), or in an extreme way (classic autism). Such a model encompasses
Lorna Wing's (1988) important notion of an autistic continuum, blurring into the normal
population.11 The work reviewed here constitutes preliminary but suggestive evidence for
the notion of male and female brain types, defined in psychometric ways. The above
psychological studies are also consistent with the claim that autism (and Asperger


11 It is tempting to surmise that children with Williams Syndrome might have an extreme form of the female brain
type, (Karmiloff-Smith et al, 1995).

38
Figure 3: Summary of the brain types.




BRAIN TYPE COGNITIVE PROFILE

The Cognitively Balanced Brain: Folk Physics = Folk Psychology

The Normal Female Brain: Folk Physics < Folk Psychology
The Normal Male Brain: Folk Physics > Folk Psychology
Asperger Syndrome Folk Physics >> Folk Psychology
Autism Folk Physics >>>Folk Psychology

39
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